1. Field of the Invention
The present invention relates to a novel placido pattern. More specifically, the novel placido pattern of the present invention allows more topographical data to be gathered from the cornea than traditional block raster or concentric ring patterns presently used on other corneal topography systems.
2. Description of Related Art
The concentric ring pattern developed by Placido in the 19th century was developed on the premise that concentric ring patterns that are reflected from a human cornea would distort based upon the anterior shape of that cornea. In the case of radical change of curvature of the cornea, such as a smaller radius or more curvature, the rings would appear to be further apart. For areas that are larger in radius, the rings would appear to be closer together. For a perfectly spherical cornea, the rings would remain concentric and evenly spaced. The difficulty with Placido""s hand held method is that the rings are hard to see due to lighting conditions and without a method of capturing the ring pattern, no review of the rings could take place.
Within the last twenty years or so, it has been found that if a recording device, such as a camera, captures these images, the captured images can be compared to an image from the reflection off of a close to perfect sphere. The differences between the two images then indicate how much the curvature of the anterior surface of the eye being tested has changed from a perfect sphere.
In addition, a mathematical relationship of elevation defined by curvature data has been developed, allowing a comparison of curvature to elevation. This means that if an image of a known radius sphere is captured and compared to an image of an unknown radius image, the elevation of the unknown surface can be calculated based upon the changes in curvature as indicated by the deviation of the rings from a perfectly circular pattern if the surface is irregular or by spacing if the surface is spherical.
The drawback to the above methodology is that for one point on an irregular cornea, the reflected rays can come from several locations on the pattern. Therefore, it is difficult to differentiate between points when the only reference circles. This becomes an issue when it is difficult to know where on a circular pattern a ray of light emanates from. This is commonly referred to as the twist angle. One attempt to deal with this problem resulted in the development of a pattern of alternating light and dark blocks arranged in a circular pattern which is disclosed in U.S. Pat. Nos. 5,841,511 and 6,213,605 to D""Souza, et al. This block pattern allows a review of the captured image vertices or corners of each block to be located and to be compared with a known good image. The vertices can be systematically located which allows one to determine which rays create which points on the image captured by the camera. In addition, an array of dark and light blocks provides better data than only concentric circles as the points of data can be located in an array of blocks because more data points can be verified than on a simple circular pattern.
It is also known to use a grid pattern for obtaining topographical data of a cornea, such as disclosed in U.S. Pat. No. 5,864,383 to Turner, et al. One problem with the use of a simple grid is that it can become difficult identifying which grid data points go with which grid intersecting lines because an essentially square pattern is being overlayed onto a spherical object. The typical method of analysis requires the center of the image to be located first. This becomes the central reference axis. This usually is aligned with the optical axis, due to the method of patient fixation. Next, each concentric ring of the pattern is located and mapped on the image. This systematic progression is more easily done on concentric type patters than with simple raster patters. When only raster grid patters are used, it becomes more difficult to systematically locate points to ensure that references to the center axis and surface points are maintained.
Therefore, it would be advantageous to have a pattern that can accommodate the twist angle problem while being easily accommodated by the software algorithms of a topographical system.